If you don't like spoilers or anything of that kind, you might want to go there straight away and explore for yourself.
Start with "About the SCP Foundation", then look at some highly rated pages.

Still here? Well, you've been warned.

The SCP Foundation is an organisation devoted to acquiring paranormal items, creatures and persons.
The goal? Make sure normal people can live a normal life.
This website gathers files about those paranormal things, designated SCPs.
SCP stands for Secure, Contain, Protect: those are the goals of the Foundation.
Each SCP has very specific containment procedures, to make sure it (or knowledge thereof) doesn't escape its containment. Obviously some of those files have been emptied of sensitive content, tagged as [DATA EXPUNGED] or blacked out.

What kind of things can you expect?
Well, first, know that there are three categories.
Safe, Euclid, Keter.
An atomic bomb is more than Safe: it won't do anything if you lock it up.
Euclid SCPs might do some weird things.
Keter are pretty much garanteed to create trouble.

An example of a "Safe" item would be SCP-914.
Stuff goes in, choose setting, stuff comes out. Definetely anomalous stuff.
This file has a huge experiment log including some fun stuff (for a rather specific definition of "fun", sure).

One example of "Euclid" is SCP-173.
It's one of the most popular of the archive, and was the major inspiration for a game called
Containment Breach.

And now to the fun part: Keter.
Those who could destroy the world, erase humanity, or reconfigure reality completely.
You'll bump into the term "XK-class End of the world scenario" quite a bit with those.
SCP-682 is a giant lizard which evolves so fast it's impossible to kill.
Have fun reading the log of the termination attempts.
SCP-231 is an example from the other end of the spectrum:
no fun to be had here, it's all about horror by omission.

Apart from the containment instructions and the incidents logs, there's another part to the website.
It's the "fanfiction" section, regrouping "Tales" which will flesh out characters from the Foundation or their ennemies,
detail complex incidents,
or just explore the possibilities of a specific SCP.
Some of them are also positively bonkers: there's a tradition of a prank war
at one the containment sites.
Prank war, dangerous paranormal items. What could possibly go wrong?

There would be loads more to say about the lore.
The Foundation's... colourful characters.
Their various ennemies.
How reading about infohazards can be enough to create a containment breach.
But of course none of this is real. Right?